Efficient transfer of lightwave signals from one waveguide to another is important to the development of optical systems and circuits. Lightwave communication systems require communication, at a minimum, between the transport medium such as optical fiber and waveguide devices such as lasers, couplers, switches and detectors. By comparison, propagating modes of the lightwave signals in the transport medium are significantly larger than in the waveguide devices. For example, signal mode optical fibers propagate lightwave signals having a fundamental mode of approximately 6-10 .mu.m width whereas waveguide devices such as single frequency semiconductor lasers support lightwave signals having a fundamental mode width less than approximately 1 .mu.m.
Tapers have been developed to control the mode size of the guided lightwave signals and, thereby, provide efficient coupling between optical waveguide devices and optical fibers. Tapers have been realized in two forms: a fiber taper which, as the name suggests, is based solely in the optical fiber and a tapered waveguide which is based solely in the device structure. In both forms, the taper increases or decreases along the direction of propagation for the lightwave signals.
Tapered waveguides, whether based in semiconductor material or electrooptic material, have been formed by epitaxial growth techniques or by special etching techniques such as photoelectrochemical etching or selective chemical etching. In addition, tapers have been formed in either the horizontal or the vertical direction, transverse to the propagation direction. For example, see Appl. Phys. Lett., 26 (6), pp. 337-40 (1975), U.S. Pat. No. 3,993,963, and U.S. Pat. No. 3,978,426. While individual attempts to create tapered waveguides have succeeded, it has been found that the techniques employed lack sufficient controllability for making identically reproducible tapered waveguides from one device to the next.